Metalworking involves the cutting or shaping of metal parts in various machining applications such as drilling, milling, turning, grinding, boring, cutting, tapping, stamping, sawing, and drawing. As part of these operations, metalworking fluids are employed to ensure that these machining operations are accomplished in an efficient manner. Metalworking fluids provide lubricity to reduce or overcome the friction that occurs as cutting tools contact the metal workpiece. They are also needed to provide cooling, in order to negate the deleterious effects of the tremendous amount of heat generated during metalworking processes. Use of metalworking fluids also facilitates prevention of the cutting tool from adhering to the metal workpiece, protection against corrosion, and removal of metal swarf from the machining area.
In conventional practice, prior art esters have been used as additives in various types of lubricant and/or metalworking fluids to provide lubricity and anti-wear characteristics, thereby enhancing the performance of the fluid. For example, it is known to use certain polyhydric alcohol esters of an aliphatic acid as a lubricity additive in a metal forming fluid. Similarly, a polyester of a dimeric acid which is either water soluble or readily emulsifiable has been used as a lubricity additive in the preparation of water-based stamping lubricants. A specific, high molecular weight polyester prepared from a polyalkylene glycol and a polycarboxylic acid has been employed as a lubricity additive in a fluid used in the manufacture and surface treatment of metallic pipe, wire, and sheet. Specific esters derived from C6 to C20 monobasic or dibasic acids and C6 to C20 primary, secondary or tertiary alcohols or blend of such alcohols have been used as lubricity additives in fluids employed for the production of two-piece metal cans. Alkoxylated Guerbet alcohols and esters for use as lubricants are also known for use in fluids.
As is known in the art, those metalworking fluids that are water-dilutable must typically be employed at a pH of about 7 to about 10 in order to prevent corrosion of the metal of the cutting tool(s) and/or of the workpiece. This alkaline pH is also necessary as a means of controlling and/or minimizing growth of microbiological organisms, which destabilize conventional metalworking fluids, thereby severely curtailing the fluid's operating life.
Unfortunately, most esters are readily decomposed (by hydrolysis and/or other chemical mechanisms) under the alkaline pH conditions in which metalworking processes are carried out. When such decomposition occurs, the effectiveness of the conventional esters as lubricity additives to the metalworking fluids is significantly degraded and the operating life of the fluid is curtailed. This is a shortfall of known prior art esters used as additives in metalworking fluids. For example, when the conventional esters added to a metalworking fluids undergo hydrolysis, the overall acidity of the metalworking fluid is increased, leading to a greater chance of corrosion of the cutting tools and/or the metal workpieces being worked. An additional concern associated with hydrolytic degradation is the generation of water-insoluble salts that can produce undesirable residues on metal parts, further destabilize the metalworking fluid, and clog the filtration systems used to maintain/recycle these fluids. Deleterious odors can also result from the decomposition of esters which can adversely affect the manufacturing environment, and increase costs by necessitating operation or installation of ventilation systems.
Attempts at devising esters that exhibit varying degrees of hydrolytic stability under various conditions have been described in the art. For example, alkanoic acid esters of cyclohexane dimethanol are used as chlorine-free extreme pressure additives. The synthesis of certain cyclohexyl esters which provide lubricity in metal-metal surface contact systems is also known. Alpha branched carboxylic acids has been described as effective lubricants in chlorine-free fluorocarbon refrigerant heat transfer fluids, particularly for the refrigerant R134a, but not as metalworking fluid lubricant.
Additionally, alkanoic acid esters derived from alpha branched carboxylic acids have been known in the prior art as useful lubricant basestocks and lubricity additives. For example, the use of polyol esters derived from five to ten carbon-containing branched and linear acids as synthetic biodegradable lubricants and functional fluids has been described, as has the preparation of certain alpha branched esters in which the two carbon chains of the alpha branched carboxylic acid contain from ten to forty-two carbon atoms, wherein each carbon chain ranges from four to twenty-two carbon atoms. These carboxylic acids can be reacted with a variety of diols and polyols to form esters which may be used as lubricant basestocks.
The use of a series of alpha branched carboxylic acid esters as lubricity additives in mold release agents which are applied either neat or as oil-in-water emulsions is known. Other researchers have disclosed the preparation of polyol ester lubricant basestocks based on linear and alpha branched carboxylic acids (such as 2-ethylhexanoic acid) combined with a small percentage of a second alpha branched carboxylic acid, isopalmitic acid.
The preparation of polyol ester blends containing specific alpha branched carboxylic acids and straight chain fatty acids which are suitable for use as hydraulic fluids and lubricant basestocks has been described. The total number of alkyl groups in the disclosed alpha branched carboxylic acid ranges from C14 to C22 and each branch can contain from C1 to C19 alkyl groups. Additionally, lubricant and power transmission fluids that are prepared in part from the 1,6-hexanediol diester of an alpha branched carboxylic acid (2-ethylhexanoic acid) are known. Also described in the prior art is the synthesis of specific decahydronaphthalene dimethanol esters which are especially useful as high temperature lubricants.
There remains a need for a hydrolytically stable ester which can provide lubricity over longer time intervals in a metalworking fluid, thereby improving the performance of and/or extending the operating life of such fluids. Such esters would enable the preparation of metalworking fluids that exhibit excellent performance over a longer operating life, thereby reducing costs while maintaining or improving the performance of the metalworking fluid over a longer period of time.